1. Field of the Invention
The invention relates to a method of correcting a deflection distortion of a picture tube by developing a correction current through a deflection yoke, the method comprising the steps of: generating a periodical waveform being synchronized to a position signal, and coupling the periodical waveform as a correction voltage to the deflection yoke to develop the correction current.
The invention also relates to a deflection correction circuit for correcting a deflection distortion of a picture tube, the correction circuit comprising: means for generating a periodical waveform from a position signal, and means for coupling the periodical waveform to a deflection yoke as a correction voltage to develop a correction current in the deflection yoke.
The invention also relates to a picture display apparatus comprising such a deflection correction circuit.
Such a deflection distortion correction may be applicable in a display system comprising a picture tube. More particular the invention is able to correct for high order deflection distortions such as North/South and convergence errors.
2. Description of the Related Art
In the market, a tendency can be observed towards more restricted specifications on N/S (North/South) raster distortion visible on a screen of a picture tube. In the ideal case, an electron beam is deflected to produce on the screen of the picture tube, a completely rectangular raster being free from non-linearity. In practice, on certain picture tubes, a specific raster distortion error can be observed called N/S seagull raster distortion. The N/S seagull distortion can be noticed as a bending of horizontal lines having more or less the shape of a gull wing. In general, the bending of the horizontal lines appears in vertical direction around the center of a line: on the upper part of the screen in downward direction, on the lower part of the screen in upward direction. If this N/S seagull raster distortion in a tube/yoke combination is too large, it can be compensated by a vertical correction current through the vertical deflection coil.
U.S. Pat. No. 4,845,409 discloses a N/S raster correction system in which a N/S seagull raster correction waveform is generated by a tuned circuit comprising a coil and a capacitor. The tuned circuit resonates at approximately twice the horizontal deflection frequency. Further, the known N/S raster correction circuit generates: a sawtooth waveform to compensate for N/S pincushion raster distortion, and a parabola waveform to compensate for N/S phase errors. The N/S seagull correction waveform, the sawtooth waveform and the parabola waveform are added together to supply a horizontal correction waveform having a repetition frequency equal to the horizontal deflection frequency. The horizontal correction waveform is multiplied by a vertical waveform to generate a composite correction waveform having the shape of the horizontal correction waveform and an amplitude depending on the vertical waveform to obtain a correction being dependent on the vertical position on the screen. The composite waveform is amplified and injected as a correction voltage in series with the vertical deflection coil. The correction voltage is integrated by the vertical deflection coil to obtain the vertical correction current for compensating the N/S distortions. The known N/S raster correction circuit generates the N/S seagull correction with a tuned circuit. A first drawback of using a tuned circuit is that the N/S seagull correction has a fixed sinusoidal shape which may not be able to compensate, to the extent desired for a non sinusoidal N/S seagull distortion of certain picture tubes. Another drawback of the tuned circuit is that the shape of the N/S seagull correction can only be changed by altering the resonance frequency of the tuned circuit, which involves cumbersome adjustment or replacement of the components used in the tuned circuit. These sinusoidal and non-sinusoidal N/S distortions are referred to as high-order N/S corrections.